Protective circuit for electron discharge devices



e 3, 1957 GEORGE YUCHT 2,815,445

. NOW BY CHANGE OF NAME GEORGE L. YOUNG EIAL PROTECTIVE cmcun' FORELECTRON DISCHARGE DEVICES Filed Jan. 16, 195

[ Inventors,

GEORGE YucA'r 3 MM: CHANGED 70 %EM, 7M.

2,815,445 Patented Dec. 3, 1957 PROTECTIVE CIRCUIT FGR ELEfiTRUNDHCHARGE DEVICES George Yucht (now by change of name George L. Young)and Russell S. Stanton, Los Angeles, (Ialili, assignors, by mesneassignments, to Hughes Aircraft Company, a corporation of DelawareApplication January 16, 1953, Serial No. 331,676

9 Claims. (Cl. 259-27) This invention relates generally to systems forprotecting electron discharge devices, and more particularly relates toa protective circuit for preventing damage to electron tubes andassociated equipment which may result from prolonged sparking withinsuch tubes.

The protective circuit of the present invention is particularly adaptedfor protecting a magnetron of the type utilized in a radar system. it isconventional practice to energize the transmitter magnetron of the radarsystem periodically by high voltage unidirectional pulses to developpulsed envelopes of the carrier frequency to be transmitted. Suchmagnetrons are subject to sparking or internal breakdown which mayresult from ionization of the residual gases in the magnetron. Duringsuch sparking, the impedance presented by the electron discharge path ofthe magnetron suddenly decreases to a low value. If the sparking orinternal breakdown of the magnetron continues for a large number ofsuccessive pulses, the magnetron and its associated equipment aredamaged.

Occasional sparking of the magnetron will not damage the tube.Consequently, it is desirable to distinguish between occasional sparkingof the magnetron and prolonged sparking which would eventually damagethe magnetron beyond repair. in order to prevent such damage, the highvoltage pulses which are periodically applied across the magnetronshould be interrupted in case of sparking that continues for any lengthof time. In accordance with the present invention, this is accomplishedby developing a voltage in response to internal breakdown of themagnetron or other electron discharge device. This voltage is impressedon a time-constant circuit including a capacitor or other charge storagemeans. The time constant or" the circuit is such that a voltage isgradually built up over a period or" time, which voltage may be utilizedto disconnect the supply from the magnetron pulser. The time constant ofthe circuit is sufficiently long so that its voltage will not build upto the value required for interrupting the pulse input to the magnetronunless the breakdown of the magnetron prevails for a predeterminedlength of time.

It is accordingly an object of the present invention to provide aprotective circuit for electron discharge devices.

Another object of the present invention is to provide a circuit forpreventing damage to pulsed magnetrons which may be caused by prolongedsparking.

A further object of the present invention is to provide a protectivecircuit for a pulsed magnetron which will deenergize the magnetron inresponse to prolonged sparking thereof, and which is unaffected byoccasional sparking which does not exceed a predetermined length oftime.

The novel features which are believed to be characteristic of theinvention, both as to its organization and method of operation, togetherwith further objects and advantages thereof, will be better understoodfrom the following description considered in connection with theaccompanying drawing in which an embodiment of the invention isillustrated by way of example, and the scope of the invention is pointedout in the appended claims. in the drawing,

Fig. l is a circuit diagram, partly in block form, of a pulsed magnetronprovided with a protective circuit in accordance with the presentinvention; and

rig. 2 is a graph illustrating voltages developed at various points ofthe circuit of Fig. 1 and plotted as a function of time.

Referring now to the drawing and particularly to Fig. 1, there isillustrated schematically a magnetron 10 including a cathode 11 and ananode 12 which may be grounded, as shown. Cathode 11 preferably isindirectly heated by a filament 13 which may be energized from asuitable source of heater supply, as shown, through a transformer 18having the midpoint of its secondary winding 19 grounded. Secondarywinding 19 is connected to heater filament 13 through a pair of windings14 and 15 which form secondary windings of a transformer 16 having aprimary winding 17. Windings 14 and 15 preferably are bifilar windings.

in accordance with conventional practice, the magnetron 10 isperiodically energized by impressing a high voltage on primary winding17 to develop a still higher negative voltage across secondary windings14 and 15. This may, for example, be done by utilizing a line-typemodulator employing a unidirectional switch, such as a thyratron, whichis periodically rendered conducting by trigger pulses impressed on thegrid thereof. Such a conventional circuit for periodically energizingthe magnetron is will now be described for providing abetterunderstanding of the protective circuit of the invention.

A positive voltage is developed by a power supply 20 having its negativeterminal grounded while its positive output terminal is connectedthrough an inductor 21 to a pulse-forming network 22 which may, forexample, be a transmission line or a simulated transmission line, asshown. Pulse-forming network 22, as illustrated, consists of atwo-terminal network including an inductor having intermediate and endtaps connected to three capacitors connected in parallel. Pulse-formingnetwork 22 is se rially connected with choke 21 between the positiveoutput terminal of high voltage supply 20 and one terminal of primarywinding 17; the other terminal of primary winding 17 may be grounded asshown.

A unidirectional switch which may, for example, be a thyratron 23, asshown, such as a hydrogen thyratron, is connected between the junctionpoint 29 of choke 21 and pulse-forming network 22, and ground. Thecathode of thyratron 23 may be directly grounded, as shown, and asuitable pulse generator 24 connected between its con trol grid andground. Thyratron 23 is arranged to be normally nonconducting and isperiodically rendered conducting by positive pulses depicted at 25 whichare developed by pulse generator 24 and impressed on the control grid ofthe thyratron. A clipper rectifier 26 has its cathode connected to thejunction point 29, while its anode is grounded through twoserially-connected resistors 27 and 23.

The circuit described thus far is conventional and its operation is wellknown. The high positive voltage developed by power supply 20 isimpressed through choke 21 on pulse-forming network 22. The voltage atthe junction point 29 may be designated V and is plotted as a functionof time in Fig. 2, to which reference is now made. Thus, curve 30 ofFig. 2 illustrates the voltage V and curve portion 31 indicates thevoltage build-up across the capacitors of pulse-forming network 22 during the interval when thyratron 23 is nonconducting. E indicates thepositive output voltage of power supply 20 tion 32 in Fig. 2. As soon asthyratron 23 begins to conduct, the voltage V drops to almost zero. Thedischarge path may be traced from pulse-forming network 22 throughthyratron 23, ground, primary winding 17, and back to pulse-formingnetwork 22. During the time in terval when thyratron 23 does notconduct, the voltage V that is, the voltage across the primary winding17 of transformer 16, which is grounded, is zero. Consequently, whenthyratron 23 begins to conduct, the voltage V across primary winding 17goes in a negative direction,

as indicated by curve portion 33 of Fig. 2. Curve portion 33 illustratesthe voltage V as a function of time, which is the voltage across primarywinding 17 as shown in Fig. 1. As soon as thyratron 23 begins toconduct, high voltage power supply is effectively disconnected frompulse-forming network 22 due to the isolation provided by choke 21.

If transformer 16 provided a perfect impedance match to the loadpresented by magnetron 1t], voltage V would never go to a negativevalue. However, as shown in Fig. 2, the voltage V may normally have asmall negative excursion shown by curve portion 34. This negativevoltage which momentarily exists at the cathode of clipper rectifier 26,renders the rectifier conducting, thus developing a small negativevoltage at the junction point of resisters 2'7, 255.

Let it now be assumed that magnetron 1t) suddently sparks due tointernal breakdown. As previously pointed out, if the sparking continuesfor any length of time, the magnetron 1t and its associated equipment,including transformer 16, pulse-forming network 22, thyratron 23, andclipper rectifier 26, may be damaged. In accordance with the presentinvention, this is prevented by a protective circuit including arectifier 36 having its cathode connected to the junction of resistors2'7, 28. The anode of rectifier 36 is connected through a currentlimiting resistor 37 to the control grid of an amplifier 38, which maybe a triode, as shown, having its cathode grounded. The anode of triode38 is connected through the winding of a relay 4% to a suitable anodevoltage supply indicated at Bl. In case the anode voltage supply 13+ isan alternating-current voltage, a capacitor 41 may be connected acrossthe winding of relay 4a to prevent chattering of the relay. Relay 41)preferably is a sensitive relay having a contact arm which will handlesmall currents only.

A capacitor 42 and a resistor are connected in parallel between thecontrol grid of triode 38 and ground. As will be more fully explainedhereafter, the resistance of resistor 37 is small compared to that ofresistor 43, so that the time constant of the RC circuit 37-42 is smallcompared to that of the RC circuit 4243.

Relay controls the energizing voltage for high voltage supply 20 bymeans of a second relay which preferably is a heavy duty relay. Asuitable source of alternating current is connected between ground andlead 45, as indicated, and lead may be connected through relay contact46 to power supply A suitable source of direct-current voltage. such asbattery 47, has its negative terminal grounded while its positiveterminal may be connected through a Contact arm 49 to a contact 48 ofrelay 44, which in turn is connected to the contact arm 51) of relay 4%.The positive terminal of battery 47 may be connected through a manualstart button 51 and lead 52 to the winding of relay the other terminalof the relay winding is grounded. Lead 52 in turn is connected tocontact 53 of relay 4t).

The high voltage supply 20 is energized in the following manner. Triode38 is normally conducting and hence the winding of relay 40 is normallyenergized. Accordingly, contact arm 50 is attracted by its relay 4i) andmakes contact with contact 53. As soon as start button 51 is momentarilydepressed, relay 44 is energized and its two movable contact arms areattracted. Consequently, the alternating current source is connected bylead 45 and contact 46 to high voltage power supply 20. When startbutton 51 is released, relay 44 remains energized through its ownholding circuit which may be traced from the positive terminal ofbattery 47 through contact 43, movable contact arm 50 of relay 40,contact 53 and through the winding of relay 44 back to ground. Hence, aslong as triode 38 conducts, high voltage power supply 25 remainsenergized, thereby to energize magnetron it periodically.

Let it now be assumed that magnetron 10 sparks. Consequently, theimpedance between its cathode 11 and anode 12 is reduced toapproximately zero resistance, and the voltage across pulse-formingnetwork 22 discharges into a substantially short-circuited load.Accordingly, the voltage V drops to a greater negative value, as shownby curve portion 55 in Fig. 2. The voltage V again builds up to apositive value as shown by curve portion 56 of Fig. 2. The largenegative voltage shown by curve portion 55 will render clipper rectifier26 conducting. The resulting voltage drop across resistors 27, 28 willdrive the voltage of the junction point of resistors 27, 28 to anegative value. Preferably, the resistance of resistor 28 is small toreduce its power dissipation so that the cathode of rectifier 36 may beoperated at a low potential with respect to the heater of the cathode.This negative voltage which is developed at the junction of resisters27, 23 will in turn render rectifier 36 conducting, and capacitor 42 ischarged to a negative potential through resistor 37. However, thevoltage across capacitor 42 which is built up in this manner isinsufficient to bias triode 33 to cut-off. As shown by curve portion 57of Fig. 2, the voltage V is relatively small; this is due to the factthat the low impedance of the sparking magnetron 10 is reflected throughsecondary windings 14, 15 into primary winding 17, which is nowsubstantially short-circuited.

As explained before, the resistance of resistor 43 is large compared tothat of resistor 37. Furthermore, the time constant of the RC network42, 43 should be large compared to the time interval between successivetrigger pulses 25. Accordingly, the charge across capacitor 42 will leakoff only to a small extent through resistor 43 between successivetrigger pulses 25.

Consequently, if magnetron lltl sparks only occasionally, the voltagebuilt up across capacitor 42 will eventually be dissipated again, sothat triode 38 is not cut-ofl. On the other hand, if sparking continuesfor a predetermined length of time determined by the time constants ofRC networks 37-42 and 42-43, the charge across capacitor 42 willeventually build up to a value sufficient to cut off triode 38.

Accordingly, the winding of relay 4th is deenergized to break theholding circuit for relay 44 which includes contact arm 50 of relay 40.Hence, the winding of relay 44 is also deenergized and thealternating-current source is disconnected from high voltage powersupply 20. Therefore, further damage to magnetron lltl, transformer 16,pulse-forming network 22, thyratron 23, clipper rectifier 26 and otherassociated equipment is effectively prevented. It is to be noted thatonce relay 44 is deenergized, it cannot be energized again until startbutton 51 is depressed. Relay 44 may be deenergized and the alternatingcurrent source for high voltage power supply 20 may be disconnected bymanually depressing stop button 60 disposed in the holding circuit ofrelay 44.

The actual circuit constants of the protective circuit of the inventionmay best be determined by experiment. T o

this end magnetron may be replaced by a resistor of low resistance andthe circuit constants adjusted until relay 40 becomes deenergized aftera predetermined length of time.

What is claimed is:

1. A protective circuit for a magnetron having a discharge path betweena cathode and an anode thereof subject to internal breakdown due tosparking, whereby the impedance of the discharge path is reduced to alow value compared to its normal value, said circuit comprising meansincluding a transformer coupled between the cathode and anode of themagnetron for periodically applying a high voltage across the dischargepath of the magnetron, a rectifier and a load impedance elementconnected across said transformer for developing a control voltage inresponse to the discharge path impedance of the magnetron said controlvoltage being at a relatively high or low vo tage level when thedischarge path impedance is at a normal or low value respectively, atime-constant network coupled across said load impedance element forintegrating said control voltage to develop an output voltagecorresponding in amplitude to the number of successive discharge pathbreakdowns, and relay means coupled to said means and to saidtime-constant network for disconnecting said means from the magnetron inresponse to said output voltage exceeding a predetermined amplitudecorresponding to a predetermined number of successive discharge pathbreakdowns.

2. A protective circuit for a magnetron having a cathode and an anode,the magnetron being subject to sparking, whereby the resistance betweenthe cathode and anode of the magnetron is reduced to a value smallcompared to its normal value, said circuit comprising a high voltagesupply, a pulse-forming network, a transformer having a primary windingconnected in series with said pulse-forming network across said highvoltage supply, said transformer having a secondary winding connectedbetween the cathode and anode of the magnetron, an electron dischargedevice connected across said high voltage supply, means coupled to saidelectron discharge device for periodically rendering said electrondischarge device conducting to develop a voltage pulse across thesecondary winding of said transformer and to render the magnetronconducting, means coupled to said high voltage supply for energizingsaid high voltage supply including a source of voltage and a relayconnected in series with said high voltage supply, a

rectifier and a load impedance element connected in series across saidhigh voltage supply, said rectifier being poled to be renderedconducting in response to sparking of the magnetron, circuit meansconnected across said load impedance element and including aresistor-capacitor network responsive to the conduction of saidrectifier to build up a control voltage thereacross, and means coupledbetween said resistor-capacitor network and said relay and responsive tosaid control voltage for deenergizing said relay to disconnect saidsource of voltage from said voltage supply in response to conduction ofsaid rectifier for a predetermined period of time corresponding to apredetermined number of sparkings of the magnetron.

3. A protective circuit for a magnetron having a cathode and an anode,the magnetron being subject to sparking, whereby the resistance betweenthe cathode and anode of the magnetron is reduced to a value smallcompared to its normal value, said circuit comprising a high voltagesupply, a pulse-forming network, a transformer having a primary Windingconnected in series with said pulse-forming network across said highvoltage supply, said transformer ha ing a secondary winding connectedbetween the cathode and anode of the magnetron, an electron dischargedevice connected across said high voltage supply, means coupled to saidelectron discharge device for periodically rendering said electrondischarge device conducting to develop a voltage pulse across thesecondary winding of said transformer and to render the magnetronconducting, means Coupled to said high voltage supply for energizingsaid high voltage supply including a source of voltage and a relay, afirst rectifier and a load impedance element connected in series acrosssaid high voltage supply, a second rectifier, a first resistor and acapacitor connected in series with said second rectifier across saidload impedance element, a second resistor connected in parallel withsaid capacitor, the resistance of said first resistor being smallcompared to that of said second resistor, said rectifiers being poled tobe rendered conducting upon sparking of the magnetron to build up acontrol voltage across said capacitor, and means coupled between saidcapacitor and said relay and responsive to said control voltage fordeenergizing said relay to disconnect said source of voltage from saidvoltage supply in response to conduction of said second rectifier for apredetermined period of time corresponding to a predetermined number ofsparkings of the magnetron.

4. A protective circuit for a magnetron having a cathode and an anode,the magnetron being subject to sparking, whereby the resistance betweenthe cathode and anode of the magnetron is reduced to a value smallcompared to its normal value, said circuit comprising a high voltagesupply, a pulse-forming network, a transformer having a primary windingconnected in series with said pulse-forming network across said highvoltage supply, said transformer having a secondary winding connectedbetween the cathode and anode of the magnetron, an electron dischargedevice connected across said high voltage supply, means coupled to saidelectron discharge device for periodically rendering said electrondischarge device conducting to develop a voltage pulse across thesecondary winding of said transformer and to render the magnetronconducting, means coupled to said high voltage supply for energizingsaid high voltage supply, a first rectifier and a load impedance elementconnected in series across said high voltage supply, a second rectifier,a first resistor and a capacitor connected in series with said secondrectifier across said load impedance element, a second resistorconnected in parallel with said capacitor, the resistance of saidresistor being small compared to that of said second resistor, saidrectifiers being poled to be rendered conducting upon sparking of themagnetron to build up a control voltage across said capacitor, and meanscoupled between said capacitor and said energizing means and responsiveto said control voltage for disconnecting said voltage supply inresponse to conduction of said second rectifier for a predeterminedperiod of time corresponding to a predetermined number of sparkings ofthe magnetron.

5 A protective circuit for a magnetron having a cathode and an anode,the magnetron being subject to sparking, whereby the resistance betweenthe cathode and anode of the magnetron is reduced to a value smallcompared to its normal value, said circuit comprising a high voltagesupply, a pulse-forming network, a transformer having a primary windingconnected in series with said pulse-forming network across said highvoltage supply, said transformer having a secondary winding connectedbetween the cathode and anode of the magnetron, a first electrondischarge device connected across said high voltage supply, meanscoupled to said first electron discharge device for periodicallyrendering said first electron discharge device conducting to develop avoltage pulse across the secondary winding of said transformer and torender the magnetron conducting, a rectifier and a load impedanceelement connected in series across said high voltage supply, saidrectifier being poled to be rendered conducting in response to sparkingof the magnetron, circuit means connected across said load impedanceelement and including a resistor-capacitor network responsive to theconduction of said rectitier to build up a control voltage thereacross,a second electron discharge device, a first relay connected in circuitwith said second electron discharge device, means coupled to said firstrelay for normally energizing said first relay and said second electrondischarge device, said second electron discharge device being coupled tosaid resistorcapacitor network for rendering said second electrondischarge device nonconducting in response to said control voltageexceeding a predetermined amplitude corresponding to a predeterminednumber of successive sparkings of the magnetron, a second relay coupledto said first relay and controlled thereby, means coupled to said secondrelay for energizing said second relay, said second relay beingdeenergized in response to deenergization of said first relay, andcircuit means for energizing said high voltage supply including a sourceof voltage and said second relay, whereby said high voltage supply isdeenergized in response to said control voltage exceeding saidpredetermined amplitude.

6. The protective circuit defined in claim 2 wherein said electrondischarge device is a thyratron.

7. The protective circuit defined in claim 2 wherein saidresistor-capacitor network has a time constant that is long enough sothat occasional sparking of the magnetron will not cause deenergizationof said relay.

8. The protective circuit defined in claim 2 wherein said it meansresponsive to said control voltage includes a further electron dischargedevice for controlling said relay.

9. The protective circuit defined in claim 2 wherein saidresistor-capacitor network includes a capacitor, a first resistorconnected in parallel to said capacitor, and a second resistor connectedserially with said capacitor across said load impedance element.

References Cited in the file of this patent UNITED STATES PATENTS2,438,962 Burlingame et a1. Apr. 6, 1948 2,548,246 Walstrorn Apr. 10,1951 2,567,744 Stanton Sept. 11, 1951 2,571,027 Garner Oct. 9, 19512,575,232 Parker et al Nov. 21, 1951 2,615,147 Hoover Oct. 21, 19522,659,008 Floyd Nov. 10, 1953 2,688,705 Fundingsland Sept. 7, 1954

